This invention relates to novel organopolysiloxane compositions capable of curing to elastic body exhibiting a strong and tight adhesion to the various surfaces of metals, glasses, ceramics and the like. In particular, this invention relates to organopolysiloxane compositions curable by the mechanism of the so-called hydrosilation reaction in which crosslinkings are formed by the addition reaction between the vinyl groups directly bonded to the silicon atoms in the organopolysiloxane molecules and the hydrogen atoms directly bonded to the silicon atoms in the organohydrogenpolysiloxane molecules in the presence of a platinum compound as the catalyst.
Known organopolysiloxane compositions curable by the mechanism of hydrosilation are, for example, compositions comprising an organohydrogenpolysiloxane, a vinyl-containing organopolysiloxane and a platinum catalyst, as disclosed in U.S. Pat. Nos. 2,915,497, 3,284,406, 3,436,366 and 3,699,073 and organopolysiloxane compositions in which the hydrogen atoms directly bonded to the silicon atoms, are in the form of a partial addition product of .alpha.-methylstyrene and an organohydrogenpolysiloxane, as disclosed in U.S. Pat. No. 3,631,220. These prior art organopolysiloxane compositions are disadvantageous because they are deficient in their adhesion especially to metallic surfaces. This lack of adhesion has tended to cause cleavages at places between the cured material and the substrate surface. Due to the possible intrusion of water into the cleavages, an adverse effect on the application of the compositions, such as, the potting of electronic circuit units, coating of electrical parts, impregnation of fly-back transformers for television sets and coating of junctions for semiconductor devices, results.